Heating device and image forming device

ABSTRACT

Within a fixing roller ( 231 ) are disposed a main heater lamp ( 234   a ) for heating a central portion of the fixing roller and a sub-heater lamp ( 235   a ) for heating opposite end portions of the fixing roller. MRnh, SRnh and ΣRnh satisfy the formula (1) or (2): 
 
Σ Rnh ≧30.5· Ln ( Ht )+382  formula (1) 
 
 MRnh ≦−21.9· Ln ( Ht )−198  formula (2), 
where MRnh is a mean value of heat distribution in a no-heat generating section of the main heater lamp; SRnh is a mean value of heat distribution in the a no-heat generating section of the sub-heater lamp; ΣRnh is the sum total of these mean values; and Ht=vp/(Mh·λ) where vp is a fixing speed (m/s), Mh a heat capacity per unit length of the heating member (J/(° C.·m)) and λ a heat conductivity of a material forming the heating member (W/(m·° C.)).

TECHNICAL FIELD

The present invention relates to a heating device for suitable use insuch devices as a fixing device of a dry electrophotographic apparatus,a drying device of a wet electrophotographic apparatus, a drying deviceof an ink-jet printer, and an erasing device for rewritable media, andto an image forming apparatus.

BACKGROUND ART

Conventionally, a fixing device, which is one of representative heatingdevices for use in such electrophotographic apparatus as a copyingmachine and a printer, generally and often has a configuration whereinheating means comprising a halogen heater or the like is disposed insidea fixing roller comprising a hollow core of aluminum or the like and thefixing device is heated to a predetermined temperature (fixingtemperature) by causing the halogen heater to generate heat.

This configuration, however, involves a problem that a length of timefrom the start of heating until the temperature of the fixing rollerreaches the predetermined fixing temperature, which is so-called warm-uptime, is long and, hence, the fixing roller need be preheated evenduring standby for the purpose of improving ease of use, thus resultingin increased power consumption during standby.

To overcome this problem, attempts have recently been made to thin thewall of the fixing roller by using an iron material having a superiorstrength to aluminum for the fixing roller in order to lower the heatcapacity of the fixing roller, thereby to shorten the warm-up time. Inthis case, however, the fixing roller has a lowered heat flow along theaxis thereof, thus raising a problem that what is called “abnormaltemperature rise at a sheet non-passage portion”, which is a phenomenonthat if recording sheets of a size smaller than the fixing roller lengthpass through the fixing roller successively then the surface temperatureof the fixing roller in a portion over which the recording sheets do notpass (sheet non-passage portion) rises abnormally, becomes easy tooccur.

To solve such a problem, such a fixing device has been proposed thatplural (mostly two) heaters for different heating regions are used toheat the different heating regions of the fixing roller selectively inaccordance with the size of a recording sheet used (see patent document1 for example).

Such a heating system employing plural heaters is basically classifiedinto the following two types. The first type comprises a combination ofa heater 234 a for heating a central region and a heater 235 c forheating the entire width region, as shown in FIG. 24 (a). When alarge-sized recording sheet is to pass through the fixing roller, onlythe entire width region heater 234 c is actuated for heating. On theother hand, when a small-sized recording sheet is to pass through thefixing roller, only the central region heater 234 a is actuated forheating.

In the first type system, however, end portions of the fixing roller 231are not supplied with heat during successive passage of small-sizedrecording sheets and, hence, the temperature thereof is lower than thatof the central portion. For this reason, if a large-sized recordingsheet is passed immediately after passage of small-sized recordingsheets, a problem of unsatisfactory fixing performance arises due to afixing failure, wrinkling, curling or the like, which occurs in edgeportions of the recording sheet.

The second type system performs heating by means of a heater (mainheater) 234 a for heating a central portion and a heater (sub-heater)235 a for heading end portions, as shown in FIG. 24(b). In this case,one temperature sensor 237 and one temperature sensor 238 are providedat the central portion and one end portion, respectively. The mainheater 234 a and the sub-heater 235 a are each controlled based on atemperature detected by a respective one of the sensor 237 at thecentral portion and the sensor 238 at the end portion.

The second type system is capable of exhibiting satisfactory fixingperformance even upon passage of a large-sized recording sheetimmediately after passage of small-sized recording sheets withoutexperiencing the aforementioned temperature drop at the end portions bycontrolling the temperature of the end portions of the fixing roller 231to an appropriate temperature by means of the sub-heater 235 a evenduring the passage of small-sized recording sheets.

Further, a method in relation to the second type system has beenproposed such that a shortcircuiting stem is inserted into a filamentcoil in a no-heat generating section of each heater to prevent theno-heat generating section from generating heat, thereby furthersuppressing the abnormal temperature rise at the sheet non-passageportion during successive passage of small-sized recording sheets (seepatent document 2 for example). Hereinafter, a heater lamp used in thistype will be referred to as a partial lamp while a heater lamp used inthe conventional type referred to as a normal lamp.

Table 1 and FIGS. 12 and 13 show the results of comparison between thecase where normal lamps were used for both of the main heater and thesub-heater in a high-speed multifunctional machine having a printingspeed of 70 cpm (pattern 1) and the case where partial lamps were usedfor both of the main heater and the sub-heater (pattern 8) as totemperature distribution along the axis of the fixing roller immediatelyafter successive passage of 100 A4- or B5R-size recording sheets. InTable 1, MRnh represents a mean value of heat distribution in theno-heat generating section of the main heater and SRnh represents a meanvalue of heat distribution in the no-heat generating section of thesub-heater. TABLE 1 Pattern 1 Pattern 2 Pattern 3 Pattern 4 Pattern 5Pattern 6 Pattern 7 Pattern 8 Main Heater Type Normal A Normal B NormalC Normal B Partial A Normal C Partial B Partial A MRnh (%) 48.0 35.930.5 35.9 13.1 30.5 26.3 13.1 Sub Heater Type Normal Normal NormalPartial Normal Partial Partial Partial SRnh (%) 36.5 36.5 36.5 14.2 36.514.2 14.2 14.2 Σ Rnh (%) 84.5 72.3 67.0 50.1 49.5 44.7 40.5 27.2

Heat distributions of respective heater lamps are shown in FIGS. 9 to 11and Table 1 in which “normal A” of FIG. 9 and “normal” of FIG. 11correspond to the main heater and the sub-heater, respectively, ofpattern 1 while “partial A” of FIG. 10 and “partial” of FIG. 11correspond to the main heater and the sub-heater, respectively, ofpattern 8.

As can be seen from FIGS. 12 and 13, the temperature uniformity obtainedwith respect to moderate A4-size sheets in the case where partial lampswere used for heater lamps (pattern 8) was comparable to that obtainedin pattern 1 employing conventional normal type lamps, and pattern 8substantially reduced the temperature rise at the sheet non-passageportion with respect to small B5R-size recording sheets as compared withpattern 1.

Patent Document 1: JP H8-220930A (paragraphs [0017] and [0018], FIGS. 1and 2)

Patent Document 2: JP 2002-258646A (paragraphs [0015] to [0021], FIGS. 1and 2)

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

However, the inventors of the present invention have found out a problemthat, in the case where partial lamps are used as heater lamps,variation in fixing roller temperature increases undesirably due tovariation in the heat distributions of the heater lamps. This problemwill be described below.

As shown in FIGS. 9 to 11, the heater lamps are each manufactured so asto provide a predetermined heat distribution. Each of heater lampsmanufactured in volume production may provide a heat distributiondeviated to a maximum of about 5 mm from a designed value due tovariation in positioning the filament during manufacture, variation inthe precision with which the heater lamps are mounted in the fixingunit, and the like.

Specifically, in cases where two types of heater lamps, i.e., mainheater and sub-heater, are used as in pattern 8, if the heatdistributions of the main heater and sub-heater are deviated to amaximum of 5 mm in opposite directions, there is a relative deviation of10 mm from one heat distribution to the other.

The results of examination of the variation in the temperature of afixing roller caused by such a deviation through an experiment usingmultifunctional machines of different printing speeds (70 cpm, 45 cpmand 26 cpm) are shown in Table 2 and FIGS. 14 and 15. Here, fourdifferent types (in material) of fixing rollers were used as seen fromTable 2. TABLE 2 Print Speed (cpm) 70 45 Fixing Speed vp (mm/s) 395 225Material Iron Iron Iron Aluminum Iron Iron Iron Aluminum Heat 45 45 45206 45 45 45 206 Conductivity λ (W/m° C.) Wall Thickness 0.70 1.30 2.201.50 0.40 0.80 1.22 0.83 t (mm) Outer Diameter 40 40 40 40 40 40 40 40 D(mm) Length L (mm) 316 316 316 316 316 316 316 316 Heat Capacity 310 567937 460 179 332 533 259 per Unit Length Mh (J/° C. · m) Temperature2.8E−05 1.5E−05 9.4E−06 4.2E−06 2.8E−05 1.5E−05 9.4E−06 4.2E−06Variation Index Ht (m³/J²) Temperature 43.7 33.4 25.0 17.5 41.7 32.626.2 16.8 Variation ΔTr (deg) Print Speed (cpm) 26 Fixing Speed vp(mm/s) 122 Material Iron Iron Iron Aluminum Heat 45 45 45 206Conductivity λ (W/m° C.) Wall Thickness 0.29 0.55 0.88 0.60 t (mm) OuterDiameter 30 30 30 30 D (mm) Length L (mm) 316 316 316 316 Heat Capacity97 183 289 141 per Unit Length Mh (J/° C. · m) Temperature 2.8E−051.5E−05 9.4E−06 4.2E−06 Variation Index Ht (m³/J²) Temperature 39.9 32.827.6 16.7 Variation ΔTr (deg)

Partial lamps were used for both of the main heater lamp and sub-heaterlamp of pattern 8. The heat distribution of the main lamp wasestablished to deviate 5 mm toward the minus side (left-hand side) fromthe sheet passage reference position (on a center registration basis),while the heat distribution of the sub-lamp established to deviate 5 mmtoward the plus side (right-hand side) from the sheet passage referenceposition (on the center registration basis).

The experiment was conducted according to a method wherein: 100 A4-sizerecording sheets were successively passed as aligned with the sheetpassage reference position on the center registration basis with thefixing roller under temperature control at a fixing temperature meetinga respective one of the printing speeds (at 210° C., 180° C. and 170° C.meeting a respective one of the printing speeds of 70 cpm, 45 cpm and 26cpm); and the temperature distribution along the axis of the fixingroller obtained after the passage of the 100 recording sheets wasdetermined using a two-dimensional radiation thermometer.

FIG. 14 is a chart showing the temperature distributions each obtainedaxially of a respective one of the different fixing rollers when theprinting speed was 70 cpm. As can be seen from FIG. 14, with the heatdistribution deviated as mentioned above, the temperature distributionsof respective fixing rollers were obtained in each of which thetemperature was higher than the controlled temperature of 210° on theminus side and lower than the controlled temperature on the plus sidewith a temperature variation ΔTr.

As can be further seen, in the case where the fixing rollers were formedfrom the same material, the fixing roller temperature variation ΔTrbecame more conspicuous with decreasing fixing roller wall thickness(=heat capacity). The heat capacity per unit length Mh(J/(° C.·m)) of afixing roller is expressed by the formula (11):Mh=Ch·Cw·π{(D/2)²−(D/2−2t)²}  formula (11)where t(mm), D(mm), Ch(J/(g° C.) and Cw(g/cm³) represent the wallthickness, outside diameter, specific heat and specific gravity,respectively, of the fixing roller.

Since the temperature variation ΔTr along the axis of a fixing roller isconsidered to become more conspicuous as the heat capacity Mh(J/(°C.·m)) or heat conductivity λ(W/(m·° C.)) of the fixing roller decreasesor as the fixing speed Vp(m/s) increases, an index indicative of thetemperature variation along the axis of the fixing roller is defined bythe formula (12):Ht=vp/(Mh·λ)  formula (12).The relationship between Ht and ΔTr was determined and the results ofthe determination were as shown in FIG. 15.

According to the determination, ΔTr and Ht have, regardless of thefixing speed, the relationship expressed by the formula (13):ΔTr=13Ln(Ht)+178  formula (13)where Ln is a natural logarithm.

Since the relationship is expressed by the approximate expression of theformula (13), Ht can be used as the index for examining ΔTr throughoutevery speed region regardless of the fixing speed. If cases where thefixing speed is 70 cpm are examined as representative cases to determinethe relationship with Ht, the relationship thus determined can beapplied to a fixing device of any fixing speed.

In a common fixing device, temperature variation factors include,besides the temperature variation ΔTr(deg) along the fixing roller axis,a temperature ripple ΔTc(deg) along the fixing roller circumferenceascribed to temperature control precision, a temperature variationΔTs(deg) due to the difference between individual temperature sensors,and like factors. Establishment need be made so that these factors fallwithin a toner non-offset region ΔTo(deg).

That is,ΔTo≧ΔTr+ΔTc+ΔTs∴ΔTr≦ΔTo−ΔTc−ΔTs  formula (14).

Since ΔTo≈40, ΔTc≈10, and ΔTs≈5 under normal circumstances, it followsthat:ΔTr≦25  formula (15).

The temperature variation ΔTr along the fixing roller axis needs tosatisfy the above-noted formulae (14) and (15).

As can be understood from FIG. 15, if a low-heat-capacity type fixingroller having Ht of not less than 7.74×10⁻⁶ is used in a system of thetype employing partial lamps like pattern 8 in order to shorten thewarm-up time, ΔTr becomes larger than 25 deg, thus causing an offset tooccur.

The present invention has been made in view of such circumstances.Accordingly, it is an object of the present invention to provide aheating device wherein the temperature distribution is rendered uniformalong the axis of a heating member having a thin wall and a low heatcapacity and comprising plural heating means thereby improving the heatefficiency of the heating device, as well as an image forming apparatusprovided with the heating device.

Solution

A heating device according to the present invention comprises:

-   -   a cylindrical heating member configured to heat and fix a toner        image carried on a recording sheet brought into contact with a        periphery of the heating member by rotation;    -   a first heating unit disposed within the heating member and        having a first heat generating section including a heating        generating portion facing a central portion of the recording        sheet, and a first no-heat generating section continuous with        the first heat generating section; and    -   a second heating unit disposed within the heating member and        having a second no-heat generating section opposed to the first        heat generating section, and a second heat generating section        opposed to the first no-heat generating section, wherein    -   assuming that: a mean value of heat distribution in the first        no-heat generating section of the first heating unit is MRnh; a        mean value of heat distribution in the second no-heat generating        section of the second heating unit is SRnh; and the sum total of        the means value of heat distribution in the first no-heat        generation section and the means value of heat distribution in        second no-heat generating section is ΣRnh(=MRnh+SRnh), MRnh,        SRnh and ΣRnh satisfy the formula (1):        ΣRnh≧30.5·Ln(Ht)+382  formula (1),        provided Ht=vp/(Mh·λ) where vp is a fixing speed (m/s), Mh a        heat capacity per unit length of the heating member (J/(° C.·m))        and λ a heat conductivity of a material forming the heating        member (W/(m·° C.)). In the formula (1), Ln is a natural        logarithm (hereinafter the same).

According to an experiment, in the case where the heating membercomprises a thin wall material and has a small heat capacity, if the sumtotal ΣRnh of means values of heat distributions in the no-heatgenerating sections of all the heating units is less than30.5·Ln(Ht)+382, the temperature variation along the fixing roller axis,which occurs when the heat distribution along the axis of each heaterlamp serving as a heating unit is deviated, becomes too large (not lessthan 25 deg), thus causing a high-temperature offset, paper wrinkling orthe like to occur. Incidentally, since the first heating unit includingthe heat generating section positioned to face the central portion of arecording sheet is heated constantly irrespective of the size of therecording sheet, the first heating unit will hereinafter be referred toas “main heating unit” as the case may be. Also, the second heatingunit, which is continuous with the first heating unit, will behereinafter referred to as “sub-heating unit” as the case may be. Thesub-heating unit is configured to heat edge portions of a recordingsheet optionally depending on the recording sheet size.

With this construction, the temperature variation along the fixingroller axis, which occurs when the heat distribution along the axis ofeach heater lamp is deviated, can be reduced to 25 deg or less byestablishing the heat distributions (relative values) of the heaterlamps serving as the heating units to satisfy the above-noted formula(1).

To realize such heat distributions, use is simply made of a heater lampof the so-called partial type having a shortcircuiting stem insertedinto the filament coil in its no-heat generating section as the heaterlamp of only one of the first and second heating units (main heatingunit and sub-heating unit) for example.

In another embodiment of the present invention, assuming that the meanvalue of heat distribution in the first no-heat generating section ofthe first heating unit is MRnh, MRnh satisfies the formula (2):MRnh≦−21.9·Ln(Ht)−198  formula (2).According to the experiment, in the case where the heating membercomprises a thin wall material and has a small heat capacity, if themean value MRnh of heat distribution in the first no-heat generatingsection of the first heating unit (main heating unit) is more than−21.9·Ln(Ht)−198, the temperature rise at the sheet non-passage portionof the fixing roller, which occurs when small-sized recording sheets arepassed through the fixing roller, becomes too large (not less than 25deg), thus causing high-temperature offset, paper wrinkling or the liketo occur.

With this feature, it is possible to prevent positional deviation of theheat distributions of the heating units and reduce the abnormaltemperature rise at the sheet non-passage portion of the fixing roller,which occurs when small-sized recording sheets are passed through thefixing roller, to 25 deg or less by establishing the heat distributions(relative values) of the heater lamps serving as the heating units tosatisfy the above-noted formula (2).

To realize such heat distributions, use is simply made of a heater lampof the so-called partial type having a shortcircuiting stem insertedinto the filament coil in its no-heat generating section as the heaterlamp of only one of the first and second heating units (main heatingunit and sub-heating unit) for example.

In yet another embodiment of the present invention, the above-notedformulae (1) and (2) are both satisfied.

With this feature, it is possible to prevent the heat distributions ofthe heating means from deviating and reduce the abnormal temperaturerise at the sheet non-passage portion of the fixing roller, which occurswhen small-sized recording sheets are passed through the fixing roller,more effectively.

In yet another embodiment of the present invention, the mean value SRnhof heat distribution in the second no-heat generating section of thesecond heating unit satisfies the formula (3):SRnh≦20%  formula (3).According to the experiment, in the case where the heating membercomprises a thin wall material and has a small heat capacity, if themean value SRnh of heat distribution in the second no-heat generatingsection of the second heating unit is more than 20%, a large differencein the power consumption of the first heating unit results between thecase where a large-sized recording sheet is passed through the fixingroller and the case where a small-sized recording sheet is passedthrough the fixing roller. For this reason, the total rated power of theheating units need be set larger. This results in a problem that anapparatus of large power consumption, such as a high-speed apparatus,cannot ensure a satisfactory temperature follow-up capability duringpassage of a small-sized recording sheet, and a like problem.

Further, in the above-described case the power consumption of the firstheating unit (main heating unit) varies largely due to the differencebetween recording sheet sizes, variation in heat distribution, or thelike. A heater lamp is most efficient when it is used at a power closeto the rated power. Under the aforementioned conditions, however, theheater lamp is used at a power considerably lower than the rated powerin fixing an image to an A4-size sheet for example, which gives rise toproblems including an increase in power consumption due to loweredthermal efficiency and a drop in fixing performance due to insufficienttemperature follow-up capability.

With the above-described feature, it is possible to reduce thedifference in the power consumption of the first heating unit (mainheating unit) between the case where a large-sized recording sheet ispassed through the fixing roller and the case where a small-sizedrecording sheet is passed through the fixing roller by establishing theheat distributions (relative values) of the heating units to satisfy theformula (3). Therefore, even an apparatus of large power consumption,such as a high-speed apparatus, can ensure a satisfactory temperaturefollow-up capability during passage of a small-sized recording sheet.Further, the difference in the power consumption of the first heatingunit due to the difference between recording sheet sizes, variation inheat distribution or the like is relatively small, which allows theheater lamp to be used at a power close to the rated power.

In yet another embodiment of the present invention, the second no-heatgenerating section of the second heating unit includes a filament coilinto which a shortcircuiting stem is inserted.

This feature can realize a stabilized heat distribution of the heatingmeans which satisfies the formula (3) by using a heater lamp of theso-called partial type having the shortcircuiting stem inserted into thefilament coil in its no-heat generating section as the heater lamp ofonly the second heating unit (sub-heating unit).

Yet another embodiment of the present invention satisfies the formula(4):Ht≧7.74×10⁻⁶  formula (4).If the heating member (fixing roller) having conventional heating units(heater lamps) in which ΣRnh is less than 30% satisfies the formula (4),the temperature variation due to the variation in heat distributionalong the axis of each of the heater lamps becomes too large (not lessthan 25 deg) irrespective of the fixing speed. However, this embodimentprevents the occurrence of such a temperature variation since ΣRnh ofthe formula (1) is established to satisfy the formula (2).

In still yet another embodiment of the present invention, the heatingmember is a heating roller comprising a cylindrical core coated with acoat layer, the core being formed from an iron material.

If the heating member (fixing roller) having conventional heating means(heater lamps) in which ΣRnh is less than 30% satisfies the formula (4),the temperature variation due to the variation in heat distributionalong the axis of each of the heater lamps becomes too large (not lessthan 25 deg) irrespective of the fixing speed. However, this embodimentin which the heating roller has a core formed from an iron materialprevents the occurrence of such a temperature variation since ΣRnh ofthe formula (1) is established to satisfy the formula (2).

ADVANTAGE OF THE INVENTION

Even with a low-heat-capacity heating member comprising a thin wallmaterial, it is possible to prevent positional deviation of heatdistributions of the heating units.

Even when small-sized recording sheets are passed, the abnormaltemperature rise at the sheet non-passage portion is suppressed and,hence, high-temperature offset, paper wrinkling and the like can beprevented from occurring.

Further, the rated power can be lowered and, hence, even an apparatus oflarge power consumption, such as a high-speed apparatus, can ensure asatisfactory temperature follow-up capability during passage ofsmall-sized recording sheets. Furthermore, the variation in the powerconsumption of the main heating means due to the difference betweenrecording sheet sizes, the variation in heat distribution or the like isrelatively small, which allows the heating means to be used at a powerclose to the rated power.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory view showing the construction of an imageforming apparatus according to one embodiment of the present invention.

FIG. 2 is an explanatory view showing the construction of an imageforming section.

FIG. 3 is an explanatory view showing the construction of a recordingmaterial feeder.

FIG. 4 is an explanatory view showing the construction of an externalrecording material feeder.

FIG. 5 is an explanatory view showing the construction of apost-processing device.

FIG. 6 is an explanatory view showing the construction of an imagereader for development.

FIG. 7 is an explanatory view showing the construction of a transportdevice for double-side printing.

FIG. 8 is an explanatory view showing the construction of a fixingdevice.

FIG. 9 is a chart showing heat distributions (relative values) of heaterlamps.

FIG. 10 is a chart showing heat distributions (relative values) ofheater lamps in another case.

FIG. 11 is a chart showing heat distributions (relative values) ofheater lamps in yet another case.

FIG. 12 is a chart showing an example of temperature distribution alongthe axis of a fixing roller in a heater lamp of a conventionalhigh-speed multifunctional machine.

FIG. 13 is a chart showing another example of temperature distributionalong the axis of the fixing roller in the heater lamp of theconventional high-speed multifunctional machine.

FIG. 14 is a chart showing temperature variation along the axis of thefixing roller caused by positional deviation of heat distribution of theheater lamp of the conventional high-speed multifunctional machine.

FIG. 15 is a chart showing the relationship between index Ht oftemperature variation along the axis of the fixing roller in the heaterlamp of the conventional high-speed multifunctional machine andtemperature variation ΔTr along the axis of the fixing roller.

FIG. 16 is a chart showing the relationship between Ht and ΔTr varyingwith positional deviation of heat distribution of heater lamps in eachof plural heat distribution patterns according to an embodiment of thepresent invention.

FIG. 17 is a chart showing the relationship between Ht and ΔTr duringpassage of small-sized recording sheets in each of the plural heatdistribution patterns according to the embodiment of the presentinvention.

FIG. 18 is a chart showing the relationship between Ht and ΣRnh varyingwith positional deviation of heat distribution and the relationshipbetween Ht and MRnh during passage of small-sized recording sheets.

FIG. 19 is a chart showing the relationship between SRnh and the maximumpower consumption and the relationship between SRnh and the powerconsumption of a main heater lamp.

FIG. 20 is a view showing heat generating section and no-heat generatingsection of each heater lamp according to embodiment 1.

FIG. 21 is a view showing heat generating section and no-heat generatingsection of each heater lamp according to embodiment 2.

FIG. 22 is an explanatory view showing the construction of a fixingdevice according to embodiment 3.

FIG. 23 is a view showing heat generating section and no-heat generatingsection of each heater lamp according to embodiment 3.

FIG. 24 is an explanatory view showing the construction of aconventional fixing device having plural heater lamps.

DESCRIPTION OF THE REFERENCE NUMERALS

-   -   23 . . . heating device    -   231 . . . heating member    -   234, 235 . . . heating means    -   234 a, 234 b . . . main heating means    -   235 a, 235 b . . . sub-heating means

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, one embodiment in which a heating device according to thepresent invention is used for the fixing device of an image formingapparatus (electrophotographic apparatus) will be described withreference to the drawings.

FIG. 1 is a sectional view showing the construction of an image formingapparatus 1. The image forming apparatus 1 includes a document imagereader 11, an image recording device 12, a recording material feeder 13,a post-processing device 14, and an external recording material feeder15. An image forming apparatus body 20, such as a digital printer,includes the image recording device 12 as an image forming section, therecording material feeder 13 as a recording material feeding section,and a transport section 17 for transporting a recording material fromthe recording material feeder 13 to a recording material deliverysection 16 via the image recording device 12.

Description will be made of the operation of the image forming apparatusbody 20. First, the document image reader 11 reads a document to obtainimage data and outputs the image data to the image recording device 12.The image recording device 12 performs appropriate image processing onthe image data inputted. The recording material feeder 13 feedssheet-shaped recording materials (recording sheets, recording mediumsand the like), such as printing sheets or OHP (Over Head Projector)sheets, one by one separately and then the transport section 17transports such recording materials to the image recording device 12through a first transport path 17 a.

The image recording device 12 forms an image on a recording materialbased on the image data by printing or the like. The recording materialprinted with the image is transported to the recording material deliverysection 16 through a second transport path 17 b and then delivered tothe outside of the apparatus.

The document image reader 11 is fitted with a document tray 18 servingas a document feeding section or a document collecting section. Whenserving as the document feeding section, the document tray 18 is capableof receiving a series of documents comprising plural pages thereon andfeeding the documents thus received to a reading section successivelywhile separating the documents one from another. When serving as thedocument collecting section, the document tray 18 receives and holdsdocuments finished with reading and successively delivered.

If printed recording materials are delivered to the recording materialdelivery section 16 in printing plural copies of a series of documentsread, mixed delivery, such as successive delivery of recording materialsprinted with the image of the same page, occurs and, hence, the user hasto sort the recording materials after printing. In view of such aninconvenience, the post-processing device 14 is connected to the imageforming apparatus body 20 so that recording materials can be deliveredto, for example, plural delivery trays 14 a and 14 b in a sorted fashionto avoid such mixed delivery.

The image forming apparatus body 20 and the post-processing device 14are spaced a predetermined distance from each other and, hence, a spaceS is defined therebetween. The image forming apparatus body 20 and thepost-processing device 14 are interconnected via an external transportsection 19. Thus, a recording material bearing an image printed thereonis transported from the transport section 17 to the post-processingdevice 14 via the external transport section 19.

From the viewpoints of energy saving and cost reduction, demand existsfor the function of printing images on both sides of a recordingmaterial such as printing paper. This function can be implemented by atransport section 21 for double-side printing which is configured toturn a recording material printed with an image on one side thereofupside down and transport it to the image forming device 12 again.

Such a recording material printed on one side thereof is turned upsidedown and transported to the image recording device 12 again by means ofthe transport section 21 for double-side printing without beingtransported to either the recording material delivery section 16 or thepost-processing device 14. The image recording device 12 prints an imageon the blank side of the recording material to implement double-sideprinting.

When it is desired to feed recording materials that exceeds the numberof types or the amount of recording materials that can be held by therecording material feeder 13, the external recording material feeder 15as a peripheral device for extension is disposed within the space S andconnected to the image forming apparatus body 20 to allow desired typesand amount of recording materials to be held and fed thereby.

Detailed description will be made of the construction of the imageforming apparatus 1. FIG. 2 is a sectional view showing the constructionof the image recording device 12. An electrophotographic processingsection in which a photosensitive drum is centered is located ongenerally left-hand side of the center of the image recording device 12.About the photosensitive drum 22 there are disposed an electrostaticcharger unit 31 for electrostatically charging the surface of thephotosensitive drum 22 uniformly, an optical scanning unit 24 forapplying a light image to the uniformly charged photosensitive drum 22by scanning to write an electrostatic latent image thereon, a developingunit 25 for developing the electrostatic latent image thus written bythe optical scanning unit 24 with use of developer, a transfer unit 26for transferring the developed image recorded on the surface of thephotosensitive drum 22 to a recording material, a cleaning unit foreliminating residual developer from the surface of the photosensitivedrum 22 to enable a fresh image to be recorded on the photosensitivedrum 22, and like components.

A fixing unit 23 is disposed above the electrophotographic processingsection. The fixing unit 23 receives recording materials bearingrespective images transferred thereto by the transfer unit 26sequentially and fixes the developer transferred to each recordingmaterial by heating. Each recording material thus printed with the imageis delivered to the recording material delivery section 16 located abovethe image recording device 12 with its printed side oriented down (facedown). Residual developer eliminated by the cleaning unit 27 iscollected and fed back to a developer feeding section 25 a of thedeveloping unit 25 for reuse.

Below the image recording device 12 is disposed the recording materialfeeding section 13 a holding recording materials and fitted within thedevice. The recording material feeding section 13 a feeds recordingmaterials one by one separately to the electrophotographic processingsection. The transport section 17 comprises plural rollers 28 and guides29. Each recording material passes through the first transport path 17 adefined between rollers, between guides, between the photosensitive drum22 and the transfer unit 25, and by like components and then, afterprinting of an image, through the second transport path 17 b definedbetween rollers and between guides and by the fixing unit 23 and thelike.

In setting recording materials in the recording material feeding section13 a, a recording material tray 30 is drawn out in the directionperpendicular to the feed direction of the image recording device 12,i.e., toward the front side in the direction perpendicular to the FIG. 2sheet face for supply or replacement of recording materials.

The underside of the image recording device 12 defines a recordingmaterial receiving section 32 for receiving recording materials fed fromthe recording material feeder 13 b (see FIG. 1) as an add-on unit andfeeding the recording materials sequentially to the space between thephotosensitive drum 22 and the transfer unit 26.

In the space around the optical scanning unit 24 are disposed a processcontrol unit (PCU) board for controlling the electrophotographicprocessing section, an interface board for receiving image data from theoutside of the apparatus, an image control unit (ICU) board forperforming predetermined image processing on image data received by theinterface board or image data read by the document image reader 11 toallow the optical scanning unit to record such image data as an image byscanning, a power source unit for supplying electric power to theseboards and units, and like components.

If connected to an external device such as a personal computer via theinterface board, the image recording device 12 alone can operate as aprinter operative to form an image on a recording material based onimage data transmitted from the external device. Though the recordingmaterial feeding section 13 a fitted within the image recording device12 is single according to the above description, more than one recordingmaterial feeding sections can be fitted within the device.

FIG. 3 is a sectional view showing the construction of the recordingmaterial feeder 13 b as an add-on unit. The recording material feeder 13b can be added on the image recording device 12 as a part of the imagerecording device 12 when the number of recording materials that can befed by the recording material feeding section 13 a is insufficient. Therecording material feeder 13 b is capable of holding recording materialsthat are larger in size than recording materials that can be held by therecording material feeding section 13 a. The recording material feeder13 b feeds recording materials held therein to a recording materialdelivery section 33 provided on an upper side thereof while separatingthe recording materials one from another.

Recording material trays 34, which are stacked in three tiers, areselectively operated under control of a CPU or the like to separatelyfeed desired recording materials held therein. Each recording materialthus fed passes from the recording material delivery section 33 to theelectrophotographic processing section through the recording materialreceiving section 32 located in a lower portion of the image recordingdevice 12. In setting recording materials in the recording materialfeeder 13 b, each recording material tray 34 is drawn out toward thefront side of the recording material feeder 13 b for supply orreplacement of recording materials.

While the recording material feeder 13 b includes three recordingmaterial trays 34 stacked in tiers according to the above description,the recording material feeder 13 b may comprise at least one or morethan three recording material trays 34 and the recording materialdelivery section 33. The recording material feeder 13 b is provided onits underside a plurality of wheels 35 which enable the image formingapparatus body 20 including the recording material feeder 13 b to moveeasily when the feeder 13 b is added on or on like occasions. The imageforming apparatus body 20 can be fixed to its installation site by meansof a stopper 36.

FIG. 4 is a sectional view showing the construction of the externalrecording material feeder 15. The external recording material feeder 15is capable of holding recording materials that exceed the number oftypes and the amount of recording materials that can be held by therecording material feeders 13 a and 13 b included in the image recordingdevice 12 and feeding the recording materials held therein one by oneseparately toward a recording material delivery section 37 located in anupper portion on the right-hand lateral side of the feeder. Eachrecording material delivered from the recording material deliverysection 37 is received by an external recording material receivingsection 38 (see FIG. 1) provided in a lower portion of the left-handlateral side of the image recording device 12.

Recording materials are set in the external recording material feeder 15through a supply port 151 defined in an upper portion of the externalrecording material feeder 15 upon supply or replacement of recordingmaterials. The supply port 151 is provided with an openable cover 152which may be configured to close the supply port 151 usually except uponsupply or replacement of recording materials. The external recordingmaterial feeder 15 is provided on its underside a plurality of wheels 39which enable the feeder 15 to move easily when the feeder 15 is to beadded on or on like occasions. The external recording material feeder 15can be fixed to its installation site by means of a stopper.

FIG. 15 is a sectional view showing the construction of thepost-processing device 14. The post-processing device 14 is spaced apredetermined distance apart from the image forming apparatus body 20(see FIG. 1). The post-processing device 14 and the image formingapparatus body 20 are interconnected via the external transport section19. Thus, a recording material bearing an image printed thereon by theimage forming apparatus body 20 is transported to the post-processingdevice 14 via the external transport section 19. The external transportsection 19 has one end connected to an external delivery section 40 (seeFIG. 2) of the image recording device 12 and the other end connected toa recording material receiving section 41 of the post-processing device14.

The post-processing device 14 has a-sort delivery section 44 capable ofdelivering recording materials transported thereto to delivery trays 14a and 14 b selectively. The sort delivery section 44 comprises pluralrollers 45 and guides 46, and a transport direction switching guide 47which can be controlled to switch one delivery destination to the other.It is possible for the user to choose one of the delivery trays 14 a and14 b as the recording material delivery destination. Thus, recordingmaterials printed with respective images can be delivered in a sortedfashion.

Examples of possible post-processing include, besides theabove-described sorting, stapling of a predetermined number of recordingmaterials, folding of printing sheets of A4 or B4 size or other size,and perforating of recording materials for filing. The post-processingdevice 14 is provided on the underside thereof with wheels 48 and 49 soas to be easily movable.

It is possible that the post-processing device 14 is provided with theexternal transport section 19 which is attachable to or detachable fromthe image recording device 12. Alternatively, the external transportsection 19 may be attachable to and detachable from both of thepost-processing device 14 and the image forming apparatus body 20.

FIG. 6 is a sectional view showing the construction of the documentimage reader 11. The document image reader 11 is operable in anautomatic reading mode for reading sheet-shaped documents automaticallyfed from an automatic document feeder (ADF) by exposure-scanning thedocument sheets one by one and in a manual reading mode for reading abook-shaped document or a sheet-shaped document which the ADF cannotfeed automatically by setting such a document manually. The image of adocument automatically or manually set on a transparent platen 50serving as a reading section is scanned by exposure to light, focused ona photoelectric converter and then converted to electric signals, togive image data. The image data thus obtained is outputted via aconnector section connected to the image recording device 12.

In reading the both sides of a document, it is possible to read imagesfrom the both sides of the document at the same time by scanning theboth sides of the document during passage of the document along thedocument feed path. For reading the lower side of a document a movableexposure-scanning optical system configured to scan the underside of theplaten is positioned stationarily at a predetermined position in thedocument feed path to guide a light image to a CCD thereby reading thedocument image.

A contact image sensor (CIS) is disposed above the document feed path toread the upper side of the document. The contact image sensor has anintegral configuration comprising a light source for exposing a documentto light, an optical lens for guiding a light image to the photoelectricconverter, and the photoelectric converter for converting the lightimage to image data. When the double-side document reading mode isselected, documents set on the document feeding section are sequentiallyfed and images on the both sides of each document under feeding are readsubstantially simultaneously.

The document image reader 11 is provided with document tray 18. Thedocument tray 18 is used in feeding documents to be read or in receivingdocuments finished with reading. In feeding documents, when documents tobe read are placed on the document tray 18, a pickup section of the ADFpicks up each document and feeds it to the platen 50. The documentfinished with reading is delivered to the outside of the reader by thedocument delivery section. In receiving documents, when documents areplaced in the document feeding section 111, the pickup section of theADF picks up each document and feeds it to the platen 50. The documentfinished with reading is delivered to the document tray 18 by thedocument delivery section.

FIG. 7 is a sectional view showing the construction of a transportdevice 21 for double-side printing. The transport device 21 has avertically oriented transport section 21 a for double-side printing andis fitted on the left-hand lateral side of the image recording device 12shown in FIG. 2. The transport section 21 a switches back and transportsa recording material delivered from the fixing unit 23 (see FIG. 2) byutilizing the delivery section 16 located in an upper portion of theimage recording device. The transport section 12 a for double-sideprinting is capable of turning each recording material upside down andfeeding the recording material to between the photosensitive drum 22 andthe transfer device 26 in the electrophotographic processing section ofthe image recording device 12 again. In the image recording device 12,the transport path for delivering each recording material toward thedelivery section 16 in the upper portion of the device is capable ofguiding a printed recording material to the post-processing device 14shown in FIG. 5 or to the transport device 21 for double-side printingby switching the recording material back.

Next, the fixing device 23 will be described in detail with reference toFIG. 8. The fixing device 23 includes a fixing roller 231 (correspondingto the heating member defined by the present invention) as an upperheating member, a pressurizing roller 232 as a lower heating member,heater lamps 234 and 235 as heat sources for the fixing roller,temperature sensors 237 and 238 forming temperature detection means fordetecting the temperature of the fixing roller 231, a cleaning roller240 in sliding contact with the pressurizing roller 232, and a controlcircuit (not shown) as temperature control means. The heater lamps 234and 235 correspond to the heating units defined by the present inventionand comprise main heater lamps 234 a and 234 b and sub-heater lamps 235a and 235 b, respectively.

The heater lamps 234 and 235, each of which comprises a halogen heater,are disposed within the fixing roller 231. When energized by the controlcircuit, the heater lamps 234 and 235 generate heat to providepredetermined heat distributions, so that the inner peripheral surfaceof the fixing roller 231 is heated by irradiation with infrared rays.

The fixing roller 231, heated to a predetermined temperature (210° C. inthis embodiment) by the heater lamps 234 and 235, serves to heatrecording sheet P formed with an unfixed toner image T passing through afixing nip zone of the fixing device. The fixing roller 231 comprises acore 231 a as a main body, and a release layer 231 b formed over theouter peripheral surface of the core 231 a for preventing toner T on therecording sheet P from offsetting.

The core 231 a comprises, for example, a metal such as iron, stainlesssteel, aluminum or copper, or an alloy thereof. In the presentembodiment, a core of iron (STKM) having a diameter of 40 mm and a wallthickness of 1.3 mm is used as the core 231 a in order to lower the heatcapacity of the core 231 a.

Fluroresins such as PFA (tetrafluoroethylene-perfluoroalkylvinyl ethercopolymer) and PTFE (polytetrafluoroethylene), silicone rubber,fluororubber, and the like are suitable for the release layer 231 b. Therelease layer 231 b is formed by coating the core 231 a with a blend ofPFA and PTFE to a thickness of 25 μm and then baking the coat. In thepresent embodiment, the rated output of the heater lamp 234 is 650 W andthat of the heater lamp 235 is 250 W.

The pressurizing roller 232 comprises a core 232 a of steel, stainlesssteel, aluminum or the like, and a heat-resistant resilient layer 232 bof silicone rubber or the like covering the outer peripheral surface ofthe core 232 a. A release layer 232 c comprising the same fluororesin asin the fixing roller may be formed over the surface of theheat-resistant resilient layer of the pressurizing roller 232. In thepresent embodiment, the pressurizing roller 232 comprises the core 232 aof stainless steel having a diameter of 40 mm, the heat-resistantresilient layer 232 b of silicone rubber having a thickness of 5 mm, andthe release layer 232 c covering the surface of the heat-resistantresilient layer 232 b, the release layer 232 c comprising a PFA tubehaving a thickness of 50 μm. The pressurizing roller 232 is pressedagainst the fixing roller 231 with a force of 745 N by means of anon-illustrated pressurizing member such as a spring thereby definingthe fixing nip zone Y having a width of about 6 mm between the fixingroller and the pressurizing roller 232.

The cleaning roller 240 serves to prevent the pressurizing roller 232from being stained with toner and paper particles by eliminating suchtoner, paper particles and the like adhering to the pressurizing roller232. Specifically, the cleaning roller 240 is pressed against thepressurizing roller 232 with a predetermined force and driven to rotatewith rotation of the pressurizing roller 232. The cleaning roller 240comprises a cylindrical metal core made of aluminum, an iron material orthe like. The present embodiment uses a stainless steel material for thecleaning roller 240.

The fixing roller 231 is provided on its peripheral surface withthermistors 237 and 238 as the temperature detection means for detectingthe surface temperature of the fixing roller. The control circuit (notshown) controls the passage of current through the heater lamps 234 and235 based on temperature data detected by each thermistor so that thetemperature of the fixing roller is held at a predetermined temperature.

Next, detailed description will be made of embodiments of heater lamps234 and 235 as the heating means.

FIG. 20 schematically illustrates the construction of the so-calledcenter registration type fixing device through which recording sheetsare to be passed with the center position of the fixing roller 231 usedas a reference wherein the heating device of the present invention isused. The heater lamps as the heating units of the fixing device includetwo heater lamps 234 a and 235 a, the heater lamp 234 a being a mainheater lamp for heating a central portion of the fixing roller, theheater lamp 235 a being a sub-heater lamp for heating the opposite endportions of the fixing roller.

The heater lamps each comprise a hollow glass tube (bulb) in whichfilament of tungsten and halogen-type inert gas are encapsulated and areeach configured to cause the filament to generate Joule heat to anelevated temperature by passing current through the filament therebyradiating infrared rays.

The heat generating section (light emitting section) of the main heaterlamp 234 a is positioned to face a central portion of each recordingsheet and extends in a region (M) which substantially coincides with asheet passage region corresponding to a B5R-size sheet which is usedparticularly-frequently among small-sized sheets and the temperaturerise at the sheet non-passage portion is most conspicuous. The heatgenerating section of the sub-heater lamp 235 a extends in regions (S1and S2) covering the no-heat generating sections (no-light emittingsections) of the main heater lamp. Thus, the region (M) extending in thecentral portion of the fixing roller is supplied with heat mainly by themain heater lamp 234 a, while the regions S1 and S2 extending inrespective of opposite end portions of the fixing roller supplied withheat by the sub-heater lamp 235 a. The no-heat generating sections areformed to be continuous with the heat generating section in the mainheater lamp 234 a, while the no-heat generating section formed to becontinuous with the heat generating sections in the sub-heater lamp 235a. The heat generating sections of the sub-heater lamp 235 a are opposedto the no-heat generating sections of the main heater lamp 234 a, whilethe no-heat generating section of the sub-heater lamp 235 a opposed tothe heat generating section of the main heater lamp 234 a.

A total of two thermistors are provided as the temperature sensors; onethermistor 237 is disposed in the heat generating section M of the mainheater 234 a and the other thermistor 238 disposed in the heatgenerating section S2 of the sub-heater lamp 235 a. A signal detected byeach thermistor is inputted to a non-illustrated control sectioncomprising a CPU. The control section is configured to control currentpassing through each heater by means of a non-illustrated driver basedon the surface temperature detected. That is, the control sectioncontrols the main heater lamp 234 a based on the output of detection bythe thermistor 237 and the sub-heater lamp 235 a based on the output ofdetection by the thermistor 238 individually and independently.

With reference to Table 1 and FIGS. 9 to 11, description will be made ofthe results of examination of heat distributions of respective of themain heater lamp 234 a and the sub-heater lamp 235 a.

The heat distributions of the main heater lamp 234 a and sub-heater lamp235 a shown in the table and figures were determined in the followingmanner. A calorimeter was placed at a location spaced apart from thecenter of each heater lamp by a distance equal to the radius of thefixing roller (20 mm in this embodiment) and caused to scan axially ofeach of the heater lamps 234 a and 235 a generating heat at theirrespective rated powers to determine a calorific value distributionalong the axis of the tube of each heater lamp. Calorific values ofdifferent portions along the axis of the tube of each heater lamp areshown as relative values (%) to the maximum calorific value, which isassumed 100%.

Here, the extent along the axis of the tube of each heater lamp whichwas subjected to measurement was the extent in which the tungstenfilament was present. In this examination, eight patterns of combinationof main heater lamp and sub-heater lamp were used to provide eightdifferent combinations of heat distributions as can be seen from FIGS. 9to 11 and Table 1. TABLE 1 Pattern 1 Pattern 2 Pattern 3 Pattern 4Pattern 5 Pattern 6 Pattern 7 Pattern 8 Main Heater Type Normal A NormalB Normal C Normal B Partial A Normal C Partial B Partial A MRnh (%) 48.035.9 30.5 35.9 13.1 30.5 26.3 13.1 Sub Heater Type Normal Normal NormalPartial Normal Partial Partial Partial SRnh (%) 36.5 36.5 36.5 14.2 36.514.2 14.2 14.2 Σ Rnh (%) 84.5 72.3 67.0 50.1 49.5 44.7 40.5 27.2

In Table 1, MRnh represents a mean value of heat distribution (relativevalue) in the no-heat generating section of the main heater lamp 234 a.Specifically, a region in which the heat distribution (relative value)is less than 70% is defined as a no-heat generating section and MRnh isan index indicative of a mean value of heat distribution (relativevalue) in the no-heat generating section.

Similarly, SRnh represents a mean value of heat distribution (relativevalue) in the no-heat generating section of the sub-heater lamp 235 a. Aregion in which the heat distribution (relative value) is less than 70%is defined as a no-heating generating portion and SRnh is an indexindicative of a mean value of heat distribution (relative value) in theno-heat generating section. It should noted that, though the centralportion and opposite end portions of the sub-heater lamp each has aregion in which the heat distribution is less than 70%, only the regionof the central portion in which the heat distribution is less than 70%is herein defined as the no-heat generating section while the regions ofthe opposite end portions in which the heat distribution is less than70% are defined as heat generating sections as shown in FIG. 11.

Further, ΣRnh is the sum total of the mean values of the heatdistributions (relative values) in respective of the no-heat generatingsections of all the heating means. Here, ΣRnh is represented by theformula (5):ΣRnh=MRnh+SRnh  formula (5).

Experiment 1

Firstly, an experiment was conducted to examine temperature variationalong the fixing roller axis in each of the combinations of heater lamps(patterns 1 to 8) due to positional deviations of respective heatdistributions of the heater lamps.

As described in the heading “PROBLEMS TO BE SOLVED BY THE INVENTION”,temperature variation ΔTr along the fixing roller axis occurs if theheat distributions of respective of the main heater lamp 234 a and thesub-heater lamp 235 a deviate from their respective established values.Patterns 1 to 8 of heater lamps were compared with each other as totemperature variation ΔTr along the fixing roller axis in the case wherethe heat distribution deviated to the maximum of 10 mm (specifically,the heat distribution of the main lamp was established to deviate 5 mmtoward the minus side (left-hand side) from the sheet passage referenceposition and the heat distribution of the sub-lamp established todeviate 5 mm toward the plus side (right-hand side) from the sheetpassage reference position.)

The experiment was conducted according to a method wherein: using threetypes of fixing rollers (any one of which was made of iron) of theshapes, characteristics and operating conditions shown in Table 3, 100A4-size recording sheets were successively passed as aligned with thesheet passage reference position determined on the center registrationbasis with each fixing roller under temperature control at 210° C.; andthe temperature distribution along the fixing roller axis obtained afterthe passage of the 100 recording sheets was determined using atwo-dimensional radiation thermometer to find ΔTr. The results of theexperiment were as shown in Table 4 and FIG. 16. TABLE 3 Print Speed(cpm) 70 Fixing Speed vp (mm/s) 395 Roller Specifications Roller 1Roller 2 Roller 3 Material Iron Iron Iron Heat Conductivity λ (W/m ° C.)45 45 45 Wall Thickness t (mm) 0.70 1.30 2.20 Outer Diameter D (mm) 4040 40 Length L (mm) 316 316 316 Heat Capacity per 310 567 937 UnitLength Mh (J/° C. · m) Temperature Variation 2.8E−05 1.5E−05 9.4E−06Index Ht (m³/J²)

TABLE 4 Heater Lump Heat Temperature ΔTr = Distribution PatternVariation ΔTr (deg) A · Ln (Ht) + B Pattern Σ Rnh (%) Roller 1 Roller 2Roller 3 A B Ht (ΔTr = 25) Pattern 1 84.45 19.0 14.5 11.0 7.2 94.26.58E−05 Pattern 2 72.33 23.3 17.9 13.3 9.0 117.9 3.41E−05 Pattern 366.95 24.0 18.6 14.0 9.0 118.7 3.16E−05 Pattern 4 50.06 30.0 23.1 17.411.3 148.6 1.83E−05 Pattern 5 49.52 30.9 23.6 17.5 12.1 157.7 1.74E−05Pattern 6 44.68 31.2 24.3 18.4 11.5 152.1 1.65E−05 Pattern 7 40.5 32.825.7 19.5 12.1 159.5 1.48E−05 Pattern 8 27.2 43.7 33.4 25.0 16.9 220.59.38E−06

Table 4 and FIG. 16 show the relationship between temperature variationΔTr along the fixing roller axis and the sum total ΣRnh of mean valuesof heat distributions (relative values) in respective of the no-heatgenerating sections of all the heater lamps and the relationship betweentemperature variation ΔTr along the fixing roller axis and index Ht oftemperature variation along the fixing roller axis.

As described in the heading “PROBLEMS TO BE SOLVED BY THE INVENTION”,Ht, as used herein, is an index defined by the formula (6):Ht=vp/(Mh·λ)  formula (6)where Mh(J/(° C.·m)) is the heat capacity of the fixing roller, λ(W/(m·°C.)) is the heat conductivity of the fixing roller, and Vp(m/s) is thefixing speed.

As can be understood from the formula (6), the index Ht of temperaturevariation along the fixing roller axis and ΔTr can be approximated toeach other regardless of the heat distribution pattern by the formula(7):ΔTr=A·Ln(Ht)+B  formula (7),and ΔTr increases with decreasing ΣRnh. With the tolerance of ΔTrestablished to be 25 deg or less, determination was conducted in each ofthe heat distribution patterns to find conditions in each of which Htsatisfied ΔTr=25 (hereinafter will be referred to as Ht(ΔTr=25)). Theresults obtained were as shown in the rightmost column of Table 4.

FIG. 18 shows the relationship between Ht(ΔTr=25) and ΣRnh. From FIG.18, the following formula (8) holds between Ht(ΔTr=25) and ΣRnh.ΣRnh=30.5·Ln(Ht)+382  formula (8)Therefore, the condition satisfying ΔTr≦25 deg is expressed by theformula (1):ΣRnh≧30.5·Ln(Ht)+382  formula (1).

Experiment 2

Next, an experiment was conducted to examine and compare temperaturerises at the sheet non-passage portion which occurred during successivepassage of small-sized paper sheets.

Specifically, as in experiment 1, experiment 2 was conducted accordingto a method wherein: using patterns 1 to 8 of heater lamps and threetypes of fixing rollers (any one of which was made of iron) of theshapes, characteristics and operating conditions shown in Table 3, 100B5R-size recording sheets were successively passed as aligned with thesheet passage reference position determined on the center registrationbasis with each fixing roller under temperature control at 210° C.; andthe temperature distribution along the fixing roller axis obtained afterthe passage of the 100 recording sheets was determined using atwo-dimensional radiation thermometer. In experiment 2 the position ofheat distribution of each heater lamp was a regular position (the centerreference position).

Table 5 and FIG. 17 show the relationship between the mean value MRnh ofheat distribution (relative values) in the no-heat generating-sectionsof the main heater lamp 234 a and the temperature variation ΔTr alongthe fixing roller axis. The relationship between MRnh and ΔTr wasexamined here because the main heater lamp 234 a is considered to be amain factor causing ΔTr to occur since the sub-heater lamp 235 a ishardly turned on during passage of small-sized sheets. TABLE 5 HeaterLump Heat Temperature ΔTr = Distribution Pattern Variation ΔTr (deg) A ·Ln (Ht) + B Pattern Σ Rnh (%) Roller 1 Roller 2 Roller 3 A B Ht (ΔTr =25) Pattern 1 48.00 40.8 28.5 18.6 20.0 250.5 1.29E−05 Pattern 3 35.8826.6 16.9 10.4 14.7 180.0 2.58E−05 Pattern 4 35.88 26.6 16.9 10.4 14.7180.0 2.58E−05 Pattern 5 30.50 26.0 16.5 9.9 14.6 178.4 2.69E−05 Pattern7 30.50 26.0 16.5 9.9 14.6 178.4 2.69E−05 Pattern 8 26.31 22.6 13.6 7.813.5 163.3 3.44E−05 Pattern 2 13.06 10.8 7.4 5.4 4.9 61.5 5.46E−04Pattern 6 13.06 10.8 7.3 5.4 4.9 61.7 5.43E−04

As can be understood from Table 5 and FIG. 17, the relationship betweenHt and ΔTr can be approximated to each other regardless of the heatdistribution pattern by the above-noted formula (7), and ΔTr increaseswith increasing MRnh. With the tolerance of ΔTr established to be 25 degor less, determination was conducted in each of the heat distributionpatterns to find conditions in each of which Ht satisfied ΔTr=25(hereinafter will be referred to as Ht (ΔTr=25)). The results obtainedwere as shown in the rightmost column of Table 5.

FIG. 18 shows the relationship between Ht(ΔTr=25) and MRnh. From FIG.18, the following formula (9) holds between Ht(ΔTr=25) and MRnh.MRnh=−21.9·Ln(Ht)−198  formula (9)

Therefore, the condition satisfying ΔTr≦25 deg is expressed by theformula (2):MRnh≦−21.9·Ln(Ht)−198  formula (2).

As can be understood from the results stated above, if the mean valueMRnh of heat distribution (relative value) in the no-heat generatingsections of the main heater lamp 234 a is more than −21.9·Ln(Ht)−198,the temperature rise at the sheet non-passage portion becomes too large(more than 25 deg), which causes the occurrence of high-temperatureoffset, paper wrinkling or the like.

Also, if the sum total ΣRnh of the mean values of the heat distributions(relative values) in respective of the no-heat generating sections ofall the heating means is less than 30.5·Ln(Ht)+382, the temperaturevariation due to variation in the heat distributions along the axes ofthe main heater lamp 234 a and sub-heat lamp 235 a becomes too large(more than 25 deg), which causes the occurrence of high-temperatureoffset, paper wrinkling or the like.

Therefore, the occurrence of high-temperature offset, paper wrinkling orthe like can be prevented even when variation occurs in the heatdistributions of respective of the main heater lamp 234 a and thesub-heater lamp 235 a if the heat distributions of respective of themain heater lamp 234 a and the sub-heater lamp 235 a are established sothat both of the formulae (1) and (2) hold.

Specific means which is capable of realizing such heat distributions isan arrangement wherein a heater lamp of the so-called partial typehaving a shortcircuiting stem inserted into the filament coil in itsno-heat generating section is used for only one of the main heater lampand the sub-heater lamp, as can be seen from Table 1.

Experiment 3

Next, power consumptions of the heat distribution patterns were examinedby comparison.

Specifically, experiment 3 was conducted according to a method wherein:using patterns 1 to 8 of heater lamps and a fixing roller comprisingroller 2 (wall thickness: 1.3 mm) shown in Table 3, the powerconsumption of each heater lamp was measured by a wattmeter with thefixing roller under temperature control at 210° C. as in experiment 1under each of the three conditions:

(1) 100 A4-size recording sheets were successively passed with the heatdistribution in the regular position;

(2) 100 A4-size recording sheets were successively passed with themaximum heat distribution deviation of 10 mm as in experiment 1; and

(3) 100 B5R-size recording sheets were successively passed with the heatdistribution in the regular position.

Table 6 and FIG. 19 show the mean power consumption of each heaterduring passage of 100 recording sheets under each condition. TABLE 6Heat Mean Power ΔTr = Main Power Distribution Sheet Consumption (W) A ·Ln (Ht) + B Consumption Pattern Position Size Main Heater Sub Heater SumMain Heater Sub Heater Sum Variation (W) Pattern 1 Regular A4 580.8246.8 827.6 645.9 276.4 922.3 78.7 Main: Normal A Max Deviation A4 567.2276.4 843.7 Sub: Normal Regular B5R 645.9 0.0 645.9 Pattern 2 Regular A4516.1 307.6 823.8 604.6 347.1 951.7 107.2 Main: Normal B Max DeviationA4 497.4 347.1 844.5 Sub: Normal Regular B5R 604.6 0.0 604.6 Pattern 3Regular A4 498.1 316.7 814.8 588.1 355.8 943.9 106.4 Main: Normal C MaxDeviation A4 481.7 355.8 837.5 Sub: Normal Regular B5R 588.1 0.0 588.1Pattern 4 Regular A4 628.4 193.1 821.5 628.4 226.5 854.8 23.8 Main:Normal B Max Deviation A4 624.5 226.5 850.9 Sub: Partial Regular B5R604.6 0.0 604.6 Pattern 5 Regular A4 396.4 426.9 823.4 481.3 462.7 944.096.6 Main: Partial A Max Deviation A4 384.7 462.7 847.4 Sub: NormalRegular B5R 481.3 96.5 577.9 Pattern 6 Regular A4 610.4 200.1 810.5610.4 233.6 844.1 22.4 Main: Normal C Max Deviation A4 608.7 233.6 842.3Sub: Partial Regular B5R 588.1 0.0 588.1 Pattern 7 Regular A4 591.1212.3 803.4 591.1 251.7 842.8 20.2 Main: Partial B Max Deviation A4586.9 251.7 838.5 Sub: Partial Regular B5R 570.9 0.0 570.9 Pattern 8Regular A4 527.3 292.4 819.7 528.1 329.6 857.7 17.2 Main: Partial A MaxDeviation A4 528.1 329.6 857.7 Sub: Partial Regular B5R 510.9 66.3 577.2

As can be seen from Table 6 and FIG. 19, the main heater lamp 234 a, forexample, in pattern 1 consumed:

-   -   580.8 W under the condition (1);    -   567.2 W under the condition (2); and    -   645.9 W under the condition (3),        and the sub-heater lamp 235 a consumed:    -   246.8 W under the condition (1);    -   276.4 W under the condition (2); and    -   0 W under the condition (3).

Thus, the maximum power consumption of the main heater lamp 234 a was645.9 W under the condition (3), the maximum power consumption of thesub-heater lamp 235 a was 276.4 W under the condition (2), and the sumtotal of the maximum power consumptions was 922.3 W. The variation inthe mean power consumption of the main heater lamp 234 a under the threeconditions was 645.9−567.2=78.7 W.

According to comparison of the heat distribution patterns as to maximumpower consumption and variation in the power consumption of the mainheater lamp 234 a, the patterns each using the partial heater for thesub-heater lamp 235 a (patterns 4, 6, 7 and 8) were lower by about 100 Win maximum power consumption than other patterns each using the normallamp for the sub-heater lamp 235 a.

When the normal lamp is used for the sub-heater lamp 235 a, the centralportion (M) of the fixing roller is supplied with heat from both of themain heater lamp 234 a and the sub-heater lamp 235 a during passage ofA4-size recording sheets because the normal-type sub-heater lamp 235 adistributes some heat even in a central portion along the fixing rolleraxis. However, small-sized sheets such as B5R-size sheets are suppliedwith heat from the main heater lamp 234 a only and, therefore, the powerconsumption of the main heater 234 a during passage of B5R-size sheetsis larger than in the case of A4-size sheets.

As a result, the variation in the power consumption of the main heaterlamp 234 a also increases to 78.7-107.2 W. This means that the mainheater 234 a is operated by a power that is lower by about 100 W thanthe rated power during passage of A4-size sheets of which size is usedmost frequently. This results in problems that: the heat exchangeefficiency of the heater lowers; and an apparatus of the type whichrequires an increased power consumption to fulfill its functions otherthan the fixing function and has a great limitation on the fixing ratedpower, such as a high-speed multifunctional machine, requires a powerexceeding the rated power during passage of small-sized sheets and hencecannot ensure a satisfactory temperature follow-up capability, and inlike problems.

On the other hand, when the partial lamp is used for the sub-heater lamp235 a, the central portion (M) of the fixing roller is supplied withheat from the main heater lamp 234 a substantially exclusively evenduring passage of A4-size recording sheets because the partial-typesub-heater lamp 235 a distributes substantially no heat in a centralportion along the fixing roller axis. For this reason, the powerconsumption of the main heater lamp 235 a varies little between the caseof passage of A4-size sheets and the case of passage of B5R-size sheets.

As a result, the variation in the power consumption of the main heaterlamp 234 a is as very small as 17.2-23.8 W. Therefore, the main heater234 a can be used at a power close to the rated power, thus assuringthat the heating member offers a superior heat exchange efficiency andhas a satisfactory temperature follow-up capability on small-sizedsheets even when used in a high-speed multifunctional machine or thelike.

It can be concluded from the above-described results that use of thepartial lamp for the sub-heater lamp 235 a is preferred to that for themain heater lamp 234 a from the viewpoint of power consumption.

With reference to FIG. 21, description will be made of embodiment 2.Since embodiment 2 is the same as embodiment 1 except heat distributionsof heater lamps, description of other features than the heater lampswill be omitted.

FIG. 21 is a view schematically showing the construction of a heatingdevice of the present invention used in a fixing device of the so-calledside registration type which is configured to pass recording sheetstherethrough with a side position of fixing roller 231 used as areference. This side registration type fixing device causes eachrecording sheet, regardless of the size thereof, to pass therethroughwith its one widthwise edge aligned with one axial end (left-hand end inthe figure) of the fixing roller 231. Even in this embodiment 2, theheat generating (light emitting) section of a main heater lamp 234 b ispositioned to face a central portion of each recording sheet.

The main heater lamp 234 b and sub-heater lamp 235 b are configured toheat a small-sized sheet passage region M and a region S, respectively,of the fixing roller 231 which extend axially of the fixing roller 231.The small-sized sheet passage region M extends from one-axial end of thefixing roller 231 and the region S is a part of a maximum sheet passageregion (M+S) other than the small-sized sheet passage region M. The mainheater lamp 234 b has a no-heat generating section formed to becontinuous with the heat generating section thereof and, similarly, thesub-heater lamp 235 b has a no-heat generating section formed to becontinuous with a heat generating section thereof. The heat generatingsection of the sub-heater lamp 235 b is opposed to the no-heatgenerating section of the main heater lamp 234 b, while the no-heatgenerating section of the sub-heater lamp 235 b opposed to the heatgenerating section of the main heater lamp 234 b.

A total of two thermistors are provided as the temperature sensors; onethermistor 237 is disposed in the heat generating section M of the mainheater 234 b and the other thermistor 238 disposed in the heatgenerating section S2 of the sub-heater lamp 235 b. A signal detected byeach thermistor is inputted to a non-illustrated control sectioncomprising a CPU. The control section is configured to control currentpassing through each heater by means of a non-illustrated driver basedon the fixing roller surface temperature detected. That is, the controlsection controls the main heater lamp 234 b based on the output of thethermistor 237 and the sub-heater lamp 235 a based on the output of thethermistor 238 individually and independently.

The main heater lamp 234 b is of the type so-called normal lamp having ano-heat generating section in which the mean value MRnh of heatdistribution (relative value) is 35.9%, while the sub-heater lamp 235 bis of the type so-called partial lamp having a no-heat generatingsection in which the mean value SRnh of heat distribution (relativevalue) is 14.2%.

By thus establishing the heat distributions of the main heater lamp 234b and sub-heater lamp 235 b, it becomes possible to reduce thetemperature variation ΔTr along the fixing roller axis due to apositional deviation of heat distribution or passage of small-sizedsheets to 25 deg or less as well as to suppress the maximum powerconsumption, the variation in the power of the main heater lamp 234 b,and the like even in the side registration type fixing device, as inembodiment 1.

With reference to FIGS. 22 and 23, description will be made ofembodiment 3 of the present invention. Since this embodiment is the sameas embodiment 1 except that an external heating roller is additionallyprovided on the pressurizing roller side, description of other featuresthan the external heating roller will be omitted. FIGS. 22 and 23schematically show the construction of a fixing device of the externalheating type to which the present invention is applied.

As shown in these figures, the fixing device 23 includes a fixing roller231 as an upper heating member, a pressurizing roller 232 as a lowerheating member, an external heating roller 233 as external heatingmeans, heater lamps 234, 235 and 236 as heat sources for the fixingroller 231 and the external heating roller 233, temperature sensors 237,238 and 239 forming temperature detection means for detecting thetemperatures of respective of the fixing roller 231 and the externalheating roller 233, a cleaning roller 240, and a control circuit (notshown) as temperature control means.

The main heater lamp 234 a is of the type so-called normal lamp having ano-heat generating section in which the mean value MRnh of heatdistribution (relative value) is 35.9%, while the sub-heater lamp 235 ais of the type so-called partial lamp having a no-heat generatingsection in which the mean value SRnh of heat distribution (relativevalue) is 14.2%.

The heater lamp 236, which comprises a halogen heater, is disposedwithin the external heating roller 233. When energized by the controlcircuit, the heater lamp 236 generates heat to provide a predeterminedheat distribution, so that the inner peripheral surface of the externalheating roller 233 is heated by irradiation with infrared rays. (In thepresent embodiment the heat generating section of the external heatercovers the entire region of the external heating roller.)

The external heating roller 233 having the heater lamp 236 as a heatingsource therewithin is positioned upstream of the fixing nip zone andconfigured to press against the pressurizing roller 232 with apredetermined pressing force. The external heating roller 233 and thepressurizing roller 232 define therebetween a heating nip zone Z (havinga heating nip width of 1 mm in this embodiment).

The external heating roller comprises a hollow cylindrical metal core233 a of aluminum, iron material or the like, and a heat-resistantrelease layer 233 b comprising a synthetic resin material that isexcellent in heat resistance and release characteristics, for example,elastomer such as silicone rubber or fluororubber, or fluororesin suchas PFA or PTFE.

The present embodiment uses a cylindrical shaft of aluminum having adiameter of 15 mm and a wall thickness of 0.75 mm as the core 233 a. Theheat-resistant release material forming the heat-resistant release layer233 b comprises a 25 μm-thick baked coat formed of a blend of PFA andPTFE. The rated output of the heater lamp 236 is 300 W.

Thermistors 234, 235 and 236 are provided on the peripheral surfaces ofrespective of the fixing roller and the external heating roller astemperature detection means for detecting the surface temperature ofeach roller. Based on temperature data detected by each of thethermistors, temperature control means (not shown) controls currentpassing through each of the heater lamps 234, 235 and 236 so that thetemperature of each roller is held at a predetermined temperature (190°C. in this embodiment).

By thus establishing the heat distributions of the main heater lamp 234a and sub-heater lamp 235 a as in embodiment 1, it becomes possible toreduce the fixing roller temperature variation ΔTr due to a positionaldeviation of heat distribution of the main heater lamp 234 a or passageof small-sized sheets to 25 deg or less as well as to suppress themaximum power consumption, the variation in the power of the main heaterlamp 234 a, and the like even in the fixing device having the externalheating roller.

It should be noted here that since the external heater 236 is not aheater having a heat generating section and a no-heat generating sectionboth, only the main heater lamp 234 a and the sub-heater lamp 235 a aretaken into account with the external heater 236 precluded in thecalculation of ΣRnh.

While any one of the foregoing embodiments comprises a combination ofone main heater lamp 234 and one sub-heater lamp 235, it is needless tosay that the present invention is applicable to a combination of onemain heater lamp 234 and plural sub-heater lamps for example.

Specifically, in the case of a fixing device including one main heaterlamp 234 and two sub-heater lamps 235, assuming that: the mean value ofheat distribution (relative value) in a no-heat generating section ofthe main heater lamp 234 is MRnh(%); the mean value of heat distribution(relative value) in a no-heat generating section of the sub-heater lamp1 is SRnh1(%); and the mean value of heat distribution (relative value)in a no-heat generating section of the sub-heater lamp 2 is SRnh2(%),the formula (10) holds.ΣRnh=MRnh+SRnh  formula (10)where SRnh=SRnh1+SRnh2.

The heat distributions of respective of the main heater lamp 234 and thetwo sub-heater lamps 235 are simply established so that the formula (10)holds while at the same time the formulae (1) and (2) hold as in theforegoing embodiments.

It should be noted that the present invention does not limit the imageforming apparatus to the construction shown in FIG. 1 and is applicableto any image forming apparatus which comprises, at least, sheet feedingmeans for feeding recording sheets, an image forming section for formingan image on each of the recording sheets fed from the sheet feedingmeans based on image data, and a heating device configured to heat andfix the image formed on the recording sheet.

1. A heating device comprising: a cylindrical heating member configuredto heat and fix a toner image carried on a recording sheet brought intocontact with a periphery of the heating member by rotation; a firstheating unit disposed within the heating member and having a first heatgenerating section including a heating generating portion facing acentral portion of the recording sheet, and a first no-heat generatingsection continuous with the first heat generating section; and a secondheating unit disposed within the heating member and having a secondno-heat generating section opposed to the first heat generating section,and a second heat generating section opposed to the first no-heatgenerating section, wherein MRnh, SRnh and ΣRnh satisfy the formula (1):ΣRnh≧30.5·Ln(Ht)+382  formula (1), where MRnh is a mean value of heatdistribution in the first no-heat generating section of the firstheating unit; SRnh is a mean value of heat distribution in the secondno-heat generating section of the second heating unit; ΣRnh(=MRnh+SRnh)is the sum total of the mean value of heat distribution in the firstno-heat generation section and the mean value of heat distribution inthe second no-heat generating section; and Ht=vp/(Mh·λ) where vp is afixing speed (m/s), Mh a heat capacity per unit length of the heatingmember (J/(° C.·m)) and λ a heat conductivity of a material forming theheating member (W/(m·° C.)).
 2. A heating device comprising: acylindrical heating member configured to heat and fix a toner imagecarried on a recording sheet brought into contact with a periphery ofthe heating member by rotation; a first heating unit disposed within theheating member and having a first heat generating section including aheating generating portion facing a central portion of the recordingsheet, and a first no-heat generating section continuous with the firstheat generating section; and a second heating unit disposed within theheating member and having a second no-heat generating section opposed tothe first heat generating section, and a second heat generating sectionopposed to the first no-heat generating section, wherein MRnh satisfiesthe formula (2):MRnh≦−21.9·Ln(Ht)−198  formula (2), where MRnh is a mean value of heatdistribution in the first no-heat generating section of the firstheating unit; and Ht=vp/(Mh·λ) where vp is a fixing speed (m/s), Mh aheat capacity per unit length of the heating member (J/(° C.·m)) and λ aheat conductivity of a material forming the heating member (W/(m·° C.)).3. A heating device comprising: a cylindrical heating member configuredto heat and fix a toner image carried on a recording sheet brought intocontact with a periphery of the heating member by rotation; a firstheating unit disposed within the heating member and having a first heatgenerating section including a heating generating portion facing acentral portion of the recording sheet, and a first no-heat generatingsection continuous with the first heat generating section; and a secondheating unit disposed within the heating member and having a secondno-heat generating section opposed to the first heat generating section,and a second heat generating section opposed to the first no-heatgenerating section, wherein MRnh, SRnh and ΣRnh satisfy the formulae (1)and (2):ΣRnh≧30.5·Ln(Ht)+382  formula (1)MRnh≦−21.9·Ln(Ht)−198  formula (2), where MRnh is a mean value of heatdistribution in the first no-heat generating section of the firstheating unit; SRnh is a mean value of heat distribution in the secondno-heat generating section of the second heating unit; ΣRnh(=MRnh+SRnh)is the sum total of the mean value of heat distribution in the firstno-heat generation section and the mean value of heat distribution inthe second no-heat generating section; and Ht=vp/(Mh·λ) where vp is afixing speed (m/s), Mh a heat capacity per unit length of the heatingmember (J/(° C.·m)) and λ a heat conductivity of a material forming theheating member (W/(m·° C.)).
 4. The heating device according to claim 1,wherein the mean value SRnh satisfies the formula (3):SRnh≦20%  formula (3).
 5. The heating device according to claim 4,wherein the second no-heat generating section of the second heating unitincludes a filament coil into which a shortcircuiting stem is inserted.6. The heating device according to claim 3, which satisfies the formula(4):Ht≧7.74×10⁻⁶  formula (4).
 7. The heating device according to claim 1,wherein the heating member is a heating roller comprising a cylindricalcore coated with a coat layer, the core being formed from an ironmaterial.
 8. An image forming apparatus comprising: sheet feeding meansfor feeding recording sheets; an image forming section for forming animage on a recording sheet fed from the sheet feeding means based onimage data; and a heating device configured to heat and fix the imageformed on the recording sheet, the heating device including: acylindrical heating member configured to heat and fix a toner imagecarried on a recording sheet brought into contact with a periphery ofthe heating member by rotation; a first heating unit disposed within theheating member and having a first heat generating section including aheating generating portion facing a central portion of the recordingsheet, and a first no-heat generating section continuous with the firstheat generating section; and a second heating unit disposed within theheating member and having a second no-heat generating section opposed tothe first heat generating section, and a second heat generating sectionopposed to the first no-heat generating section, wherein MRnh, SRnh andΣRnh satisfy the formula (1):ΣRnh≧30.5·Ln(Ht)+382  formula (1), where MRnh is a mean value of heatdistribution in the first no-heat generating section of the firstheating unit; SRnh is a mean value of heat distribution in the secondno-heat generating section of the second heating unit; ΣRnh(=MRnh+SRnh)is the sum total of the mean value of heat distribution in the firstno-heat generation section and the mean value of heat distribution inthe second no-heat generating section; and Ht=vp/(Mh·λ) where vp is afixing speed (m/s), Mh a heat capacity per unit length of the heatingmember (J/(° C.·m)) and λ a heat conductivity of a material forming theheating member (W/·(m·° C.)).